Presented at Short Course IV on Exploration for Geothermal Resources,

Presented at Short Course IV on Exploration for Geothermal Resources,
organized by UNU-GTP, KenGen and GDC, at Lake Naivasha, Kenya, November 1-22, 2009.
Kenya Electricity Generating Co., Ltd.
GEOTHERMAL TRAINING PROGRAMME
Geothermal Development Company
GEOTHERMAL ENERGY UTILISATION
Martha Mburu
Geothermal Development Company
P. O. Box 100746, Nairobi 00101
KENYA
mmburu@gdc.co.ke
ABSTRACT
Geothermal energy is energy derived from the natural heat of the earth. The earth’s
temperature varies widely, and geothermal energy is usable for a wide range of
temperatures from room temperature to well over 300°C. Geothermal energy can
be used for both electricity generation and direct uses depending on the
temperature and chemistry of the resources. High to medium resources are used for
electricity generation while medium to low resources are is mainly used for direct
application. For efficient utilization of resources, geothermal energy is being
utilized for combined heat and power. Since geothermal energy is renewable,
indigenous and environmentally benign, and can be used as base load, its will be
the future source of energy in Kenya and most of the Eastern African countries
especially those that are located within the east Africa Rift. This will reduce uses of
fossil fuels and hence address global warming.
1. INTRODUCTION
The word geothermal was developed from two Greek words, “geo”, meaning earth, and “thermos”,
meaning heat since it is power extracted from heat stored in the earth. This geothermal energy
originates from radioactive decay of minerals, and from solar energy absorbed at the surface.
Geothermal heat, is estimated to be about 5,500oC at the Earth’s core – about as hot as the surface of
the sun. This energy been used for bathing since paleolithic times and for space heating since ancient
roman times. Nowadays, it is used commercially for both generating electricity and direct uses.
Worldwide, geothermal plants have the capacity to generate about 10 GW of electricity as of 2007,
and in practice supply 0.3% of global electricity demand. An additional 28 GW of direct geothermal
heating capacity is installed for district heating, space heating, hot spas, industrial processes,
desalination and agricultural applications (Fridleifsson et. al, 2008).
Because of the near limitless ability of the earth to produce magma, and the continuous transfer of heat
between subsurface rock and water, geothermal energy is considered a renewable resource. It is also
cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to
areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the
range and size of viable resources, especially for applications such as home heating, opening a
potential for widespread exploitation. Geothermal power has the potential to help mitigate global
warming if widely deployed in place of fossil fuels.
2. UTILISATION OF GEOTHERMAL ENERGY
The heat from the earth's own molten core is conducted to the adjacent rocks and eventually is
transferred to underground water reservoirs through convection. Steam/water heated by the geothermal
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Geothermal energy utilisation
heat can be tapped using different technologies and channeled to various uses. Utilisation of
geothermal fluid depends heavily on its thermodynamic characteristics and chemistry. These factors
are determined by the geothermal system from which the fluid originated. Geothermal fluids have
been classified differently by different authors. Some authors have done so by using temperatures
while others have used enthalpy (Dickson and Fanelli, 2004). The most common criterion is that based
on enthalpy. The resources are divided into low, medium and high enthalpy resources. Table 1 shows
the classifications proposed by a number of authors.
TABLE 1: Classification of geothermal resources (oC) (Dickson and Fanelli, 2004)
a) Muffler and Cataldi (1978); b) Hochstein (1990); c) Benderitter and Cormy (1990);
d) Nicholson (1993);
e) Axelsson and Gunnlaugsson (2000).
Depending on the enthalpy, geothermal fluid can be utilised either for electricity generation or direct
applications. Electricity generation is the most important form of utilization of high-temperature
geothermal resources while low to medium resources are better suited for non-electric (direct)
application (Table 2).
TABLE 2: Basic technology commonly used for geothermal energy
Reservoir temperature
High temperature, >220°C
Intermediate temperature,
100-220°C
Low temperature, 30-150°C
Reservoir
Common use
Technology commonly chosen
fluid
Water or Power generation
Flash steam; Combined (flash
Steam
and binary) cycle
Direct use
Direct fluid use; Heat exchangers;
Heat pumps
Water
Power generation
Binary cycle
Direct use
Direct fluid use; Heat exchangers;
Heat pumps
Water
Direct use
Direct fluid use; Heat exchangers;
Heat pumps
2.1 Electricity generation from geothermal energy
Geothermal energy is utilized in many parts of the world especially those that are located on plate
boundaries or tectonically active regions (Figure 1). In 2005, 24 countries generated a total of 56,786
GW-hours (GWh) (204 PJ) of electricity from geothermal power, accounting for 0.3% of worldwide
electricity consumption. Output is growing by 3% annually because of a growing number of plants and
improvements in their capacity factors, development of binary cycle power plants and improvements
in drilling and extraction technology. Since geothermal power does not rely on variable sources of
energy, unlike, for example, wind or solar, its capacity factor can be quite large – up to 96% has been
demonstrated (Lund, 2003). The global average was 73% in 2005 (Fridleifsson et. al, 2008).
Geothermal power plants are categorized in three main ways depending on the fluid temperature,
pressures and chemistry.
•
Condensing power plants (dry steam, single or double flash systems)
Geothermal energy utilisation
•
•
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Back-pressure turbines (release to the atmosphere)
Binary plants (for lower temperature or separated brine).
FIGURE 1: Plate and tectonic boundaries
The largest group of geothermal power plants in the world is located at The Geysers, field in
California, United States As of 2004, five countries (El Salvador, Kenya, the Philippines, Iceland, and
Costa Rica) generate more than 15% of their electricity from geothermal sources (WEA, 2004).
Geothermal resources vary in temperature from 30-350 o C, and can either be dry steam, two-phase (a
mixture of steam and water) or just liquid water. In order to extract geothermal heat from the earth,
water is the transfer medium. Naturally occurring groundwater is available for this task in most places
but more recently technologies are being developed to even extract the energy from hot dry rock
resources. The temperature of the resource is a major determinant of the type of technologies required
to extract the heat and the uses to which it can be put. Table 3 lists the basic technologies normally
utilised according to resource temperature and fluid availability.
TABLE 3: Geothermal energy applications
Electricity production
Wells drilled into a geothermal reservoir produce hot water and
steam from depths of up to 3 km. The geothermal energy is
converted at a power plant into electric energy, or electricity.
Hot water and steam are the carriers of the geothermal energy.
Direct use
Applications that use hot water from geothermal resources
directly. Examples: space heating, crop & lumber drying, food
preparation, aquaculture, industrial processes, etc. Historical
traces back to ancient Roman times, e.g. for baths
Geothermal heat pumps
Taking advantage of relatively constant earth temperature as the
source and sink of heat for both heating and cooling, as well as hot
water provision. One of the most efficient heating and cooling
systems available.
Hot Dry Rock
Deep geothermal/EGS
(both not commercial yet)
Extracts heat by creating a subsurface fracture system to which
water can be added through injection wells. Water is heated by
contact with the rock and returns to the surface through production
wells. Energy is then converted at a power plant into electric
energy as in a hydrothermal geothermal system.
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Geothermal energy utilisation
High temperature geothermal reservoirs containing water and/or steam can provide steam to directly
drive steam turbines and electrical generation plant. More recently developed binary power plant
technologies enables more of the heat from the resource to be utilised for power generation. A
combination of conventional flash and binary cycle technology is becoming increasingly popular.
High temperature resources commonly produce either steam, or a mixture of steam and water from the
production wells. The steam and water is separated in a pressure vessel (Separator), with the steam
piped to the power station where it drives one or more steam turbines to produce electric power. The
separated geothermal water (brine) is either utilised in a binary cycle type plant to produce more
power, or is disposed of back into the reservoir through deep (injection) wells. The following is a brief
description of each of the technologies most commonly used to utilise high temperature resources for
power generation.
2.1.1 Dry steam power plants
Power plants using dry steam systems were the first type of geothermal power generation plants built.
They use steam from the geothermal reservoir as it comes from wells and route it directly through
turbine/generator units to produce electricity (Figure 2). An example of a dry steam generation
operation is at the Geysers in northern California.
FIGURE 2: Schematic of a dry steam power plant.
2.1.2
Flash Steam Power Plant
Flash steam power plants (Figure 3) are the most common type of geothermal power plant. The steam,
once it has been separated from the water, is piped to the powerhouse where it is used to drive the
steam turbine. The steam is condensed after leaving the turbine, creating a partial vacuum and thereby
maximizing the power generated by the turbine-generator. The steam is usually condensed either in a
direct contact condenser, or a heat exchanger type condenser. In a direct contact condenser the cooling
water from the cooling tower is sprayed onto and mixes with the steam. The condensed steam then
forms part of the cooling water circuit, and a substantial portion is subsequently evaporated and is
dispersed into the atmosphere through the cooling tower. Excess cooling water called blow down is
often disposed of in shallow injection wells. As an alternative to direct contact condensers shell and
tube type condensers are sometimes used, as is shown in the schematic below. In this type of plant, the
condensed steam does not come into contact with the cooling water, and is disposed of in injection
wells.
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FIGURE 3: Schematic of a single flash steam power plant
Typically, flash condensing geothermal power plants vary in size from 5 MW to over 100 MW.
Depending on the steam characteristics, gas content, pressures, and power plant design, between 6000
kg and 9000 kg of steam each hour is required to produce each MW of electrical power. Small power
plants (less than 10 MW) are often called well head units as they only require the steam of one well
and are located adjacent to the well on the drilling pad in order to reduce pipeline costs. Often such
well head units do not have a condenser, and are called backpressure units. They are very cheap and
simple to install, but are inefficient (typically 10-20 tonne per hour of steam for every MW of
electricity) and can have higher environmental impacts.
2.1.3 Binary cycle power plants
In reservoirs where
temperatures
are
typically less than
220o C. but greater
than 100o C binary
cycle plants are often
utilised. The illustration
(Figure
4)
shows the principal
elements of this type
of
plant.
The
reservoir fluid (either
steam or water or
both)
is
passed
through
a
heat
exchanger
which
heats a secondary
working
fluid
(organic) which has
FIGURE 4. Power schematic of a binary cycle power plant
a boiling point lower
than 100o C. This is typically an organic fluid such as Isopentane, which is vaporised and is used to
drive the turbine. The organic fluid is then condensed in a similar manner to the steam in the flash
power plant described above, except that a shell and tube type condenser rather than direct contact is
used. The fluid in a binary plant is recycled back to the heat exchanger and forms a closed loop. The
cooled reservoir fluid is again re-injected back into the reservoir.
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Geothermal energy utilisation
Binary cycle type plants are usually between 7 and 12 % efficient, depending on the temperature of the
primary (geothermal) fluid. Binary Cycle plant typically vary in size from 500 kW to 10 MW.
2.2 Direct uses of geothermal energy
Low to medium temperature geothermal resources are utilized for direct uses or binary power plants.
The oldest and probably best known direct application of geothermal energy is in baths and spas, used
during Roman times. Other more coordinated and commercial uses include district heating systems,
like those found in Iceland which are the largest geothermal district heating system in the world,
tourist centres thermal baths in Hungary and China; geothermally heated fish farms and food
processing and de-hydration in Greece and the USA. Furthermore the possibility to utilize ground
source (geothermal) heat pumps to either heat homes directly or through energy efficiency programs,
allowing savings in energy costs, has vast possibilities for individual or industrial use.
All those "direct use" applications utilize geothermal energy produced from lower temperature water
of less than 150 degrees centigrade that is being derived from wells of 100-3,000 m deep wells. Today,
around 73 nations utilize geothermal energy directly, with an overall energy output of 75.9 TWh
thermal per year. The number of nations using geothermal heat is growing constantly.
Approximately 70 countries made direct use of a total of 270 PJ of geothermal heating in 2004. More
than half of this energy is being used for space heating, and another third for heated pools. The
remainder supported industrial and agricultural applications. The global installed capacity was 28 GW,
but capacity factors tend to be low (30% on average) since heat is mostly needed in the winter. The
above figures are dominated by 88 PJ of space heating extracted by an estimated 1.3 million
geothermal heat pumps with a total capacity of 15 GW. Heat pumps for are the fastest-growing means
of exploiting geothermal energy, with a global annual growth rate of 30% in energy production. Most
of these new heat pumps are being installed for home heating.
Direct heating in all its forms is far more efficient than electricity generation and places less
demanding temperature requirements on the heat resource. Heat may come from co-generation with a
geothermal electrical plant or from smaller wells or heat exchangers buried in shallow ground. As a
result, geothermal heating is economical over a much greater geographical range than geothermal
electricity. Where natural hot springs are available, the heated water can be piped directly into
radiators. If the ground is hot but dry, earth tubes or downhole heat exchangers can collect the heat.
But even in areas where the ground is colder than room temperature, heat can still be extracted with a
geothermal heat pump more cost-effectively and cleanly than it can be produced by conventional
furnaces (Lund, 2006). These devices draw on much shallower and colder resources than traditional
geothermal techniques, and they frequently combine a variety of other functions, including air
conditioning, seasonal energy storage, solar energy collection, and electric heating. Geothermal heat
pumps can be used for space heating essentially anywhere in the world.
Geothermal heat supports many applications. In Iceland, Reykjavík and Akureyri pipe hot water from
geothermal plants below pavement to melt snow. District heating applications use networks of piped
hot water to heat buildings in whole communities. Geothermal desalination has been demonstrated.
Some of the main direct uses of geothermal energy are
3. GEOTHERMAL UTILISATION IN KENYA
Kenya has a large potential of geothermal energy estimated to be more than 4000 MWe (Omenda and
Simiyu, 2006). Currently, only 4% of this energy is being exploited primarily for electricity
generation. Other non electric applications have not been given the necessary attention (Mwangi,
2005). There are a number of potential applications of geothermal energy in Kenya.
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3.1 Electricity generation using geothermal energy in Kenya
Geothermal energy exploitation in Kenya has been primarily for electricity generation and it
constitutes 14 % of the country’s electricity. KenGen generates 89% of this energy from two power
plants. Olkaria I power plant was commissioned in the year 1981 while Olkaria II (Figure 5) was
commissioned in the year 2003. Drilling for a third power plant, Olkaria IV is currently ongoing for a
140 MWe power plant expected in 2012.
FIGURE 5: Olkaria II power station, commissioned in 2003
OrPower 4, Inc., an independent power producer (IPP) is currently generating 48 MWe, 12 MWe
from an Ormat binary plant commissioned in the year 2000 and 36 MWe is from a single flash plant
commissioned in 2009. Oserian Development Company, (Oserian), constructed a 1.8 MWe binary
plant Ormat OEC in 2004 and 2MWe from a back pressure turbine commisioned in 2007. Both of
these plant use wells leased from KenGen (Knight et al, 2005).
3.2
Direct utilisation of geothermal energy for in Kenya
The only commercial application of geothermal energy for other uses other than electricity generation
(direct applications) in Kenya is at Oserian. Since 2003, the firm has been utilising energy from a
geothermal well OW-101 leased from KenGen for greenhouse heating. The system started by heating
3 hectares and has been expanded to 50 hectares (Figure 6). Other minor geothermal direct uses are at
Lake Borogia hotel, where a naturally occurring geothermal hot spring is being used to warm a
swimming pool and at Eburru geothermal resource where the local community use geothermal energy
for drying pyrethrum products (Mwangi and Mburu, 2005). Other potential applications include
greenhouse farming, domestic hot water supply, animal husbandly, and balneology among others.
Hot springs have been used to heat spas in tourist hotels for example in Borogia hotel which is located
near the Bogoria prospect The Local community at Eburru geothermal resource condenses the steam
from fumarole and uses the water for domestic purposes. They also use geothermally heated driers to
dry their pyrethrum products. A study carried out in 2008 (Mburu, 2008) revealed potential market for
direct utilisation in Kenya. Some of the main potential uses.
Although there is great potential for both electricity generation and direct utilisation of geothermal
energy in the Kenyan Rift valley, geothemal energy has been primarly used for electricity generation
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Geothermal energy utilisation
(Mwangi, 2005). Investigations into possible applications of this energy for direct applications show
potential in swimming pools and spas, balneology, drying of farm produce and greenhouse heating
(Lagat, 2006). This study will also investigate the feasibility of some of these applications together
with domestic hot water supply and space cooling.
Pipes carrying
hot water
FIGURE 6: Geothermal greenhouse heating at Oserian Company, Kenya.
3.2.1 Greenhouse heating
Heating of greenhouses using geothermal energy has been practiced in many countries in the world
with encouraging economic, social and environmental benefits (Lund and Freeston, 2000).
In 2003, Oserian Development Company Ltd, a private firm growing rose flowers for export leased a
geothermal well from KenGen for use in greenhouse heating. Steam and brine from this well are being
used in a heat exchanger to supply hot water to heat 50 hectares of the greenhouses. In Kenya and
other warmer countries,greenhouse heating is done primarily for humidity control since, increase in
humidity results in mutiplication of fungus affecing the crops. Heating also enhances growth and saves
on fuel costs that would be incured if heating is to be done using fossil fuels (Hole and Mills, 2003).
3.2.2 Domestic hot water uses
District heating and domestic hot water supply using geothermal energy is dominant in many cold
European countries. Though most of them use ground source heat pumps, those which have large
geothermal resources like Iceland use geothermal energy (Fridleifsson, 2001).
The various uses for domestic hot water include dish washing, laundry, bathing and hand washing. Hot
water consumption depends on uses and application temperature (Yao et. al., 2003). According to
BRE, 2008, domestic hot water should be supplied at 60oC to avoid multiplication of Legionnaire
bacteria (Figure 7) and to avoid scalding.
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Figure 7: Safe temperature for domestic hot water (AWT, 2003)
3.2.3 Leisure therapeutic hot water uses
Water at elevated temperatures has been used for leisure and for its therapeutic potential for many
centuries. Geothermal waters have in particular been used widely for this purpose. In Iceland for
example, many tourist centres have geothermally heated pools, spas and steam bath. Blue lagoon,
which uses geothermal brine from Svatsengi geothermal power plant, is a major tourist attraction with
about 300,000 per year (Sigurgeirsson and Olafsson, 2003).
For safety and hygiene of swimmers, swimming pool water requires continuous cleaning and filtering
at a circulation rate of between 4-8 hours. This depends on size and usage of the pool. A conventional
commercial swimming pool 25m length, 10m width and average height of 1.5 m requires a circulation
time of 6 hours (Dayliff, 2006). Water temperature at the swimming pool ranges between 26-30oC
(Seneviratne, 2007). Saunas temperature must be between 80oC and 100oC while steam baths are
requires water from 43-60oC (Hillingdon, 2008).
3.2.4
Crop drying
Geothermal energy has been used to dry vegetables, fruits wheat and other cereals (Lund et al, 2005).
Due to high solar radiation in Kenya, crops have been traditionally dried using solar energy but
farmers incur heavy crop losses due to these unreliability of solar energy during rainy and cold seasons
and at night. Alternative energy sources have to be used during such times. Farmers in Eburru Kenya,
use geothermal energy to dry pyrethrum flowers (Mwangi, 2005). However, this is uncoordinated and
is on a very small scale. When such activities are co-ordinated, they can be a major benefit to the
farmers of the area and to the environment.
4. CONCLUSIONS
Geothermal energy has been utilized for direct uses for along time in human history especially where
the energy has been occurring naturally for example in Romans, Chinese, Ottomans and the Japanese.
This was mainly in bathing and on small-scales. Currently, direct uses are on commercial level and are
replacing fossil fuels in uses of low heat applications such as district heating, processing of
agricultural products, therapeutic uses as well as in aquaculture.
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Geothermal energy utilisation
The earliest industrial exploitation began in the 19th century with the use of geyser steam to extract
boric acid from volcanic mud in Larderello, Italy. In the 20th century, demand for electricity led to the
consideration of geothermal power as a generating source. In 1904, at the same Larderello dry steam
field where geothermal acid extraction began, electricity generation was begun. Cierro Prietto in
Larderello is the world's first commercial geothermal power plant. It was the world's only industrial
producer of geothermal electricity until New Zealand built a plant in 1958. These two plants are
operational to date.
Depending on the temperature, pressure and chemistry of geothermal resources, geothermal energy
can be used in electricity generation using either conventional or binary power plants. In Kenya for
example, 167 MWe is used for electricity generation using condensing, binary and back-pressure
turbines.
The African Rift has a geothermal potential of more than 10,000 MWe and much more thermal
energy. This energy if utilized can go a long way to replacing uses of fossil fuels and hence address
the global warming.
Where temperature and pressure allow, combined heat and power should be adopted in all future
geothermal developments. This will enhance efficient utilization of geothermal resources. This calls
for research to explore potential direct uses especially among the communities living in geothermal
resource area.
Geothermal energy has been used for direct applications in many parts of the world such as Iceland,
USA, Turkey, China and many European countries. This has proved to be very economical and
reliable since geothermal energy is has very high availability (>90%). In Africa, geothermal energy
has not been widely used for direct uses mostly due to absence of cold seasons which requires
applications low heat. This is however changing as the prices of fossil fuels continue to increase and as
more and more focus goes to the utilisation of renewable energy sources to address the climate
change. Geothermal energy can be used to replace fossil fuels in greenhouse heating, aquaculture and
crop drying as well as in leisure and therapeutic uses. 16 MWt is used for direct applications in
greenhouse heating and a limited amount of thermal energy is used for low heat energy applications in
crop drying and swimming.
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